The invention relates to a device for producing a thin steel strip, comprising at least one or more continuous-casting machines for casting thin steel slabs, a furnace device which is suitable for heating and/or homogenizing a slab, and at least one rolling device for reducing the thickness of a slab which is conveyed out of the furnace device.
The invention also relates to a process for producing a steel strip, in which liquid steel is cast in at least one continuous-casting machine to form a slab and, utilizing the casting heat, is conveyed through a furnace device and, in a rolling device, is rolled to form the strip with a desired final thickness.
A device of this nature is described in application WO-A-97/46332. By this reference, the contents of this application are deemed to be incorporated into the present application. The said application proposes, inter alia, to use a device of this nature for an endless rolling process. In the said application, an endless rolling process is understood to mean a rolling process in which slabs or, following passage through a preliminary rolling device, strips are coupled together so that an endless rolling process can be carried out in a finishing mill.
In the past, it has been proposed to couple slabs together by providing the end of one slab with a shape which is such that it can be coupled to the front edge of a following slab which is also provided with a suitable, often complementary, shape. The devices which are required to do this are highly complicated and take up considerable amounts of space. In addition, the slabs which are to be coupled together are exposed to the atmosphere for a considerable length of time, with the result that the slabs cool down and a layer of oxide is formed on the slabs.
An endless rolling process, in particular when applied to thin-cast slabs, i.e. slabs with a thickness of 100 mm or less, preferably 80 mm or less, provides the possibility of achieving a very high level of temperature homogeneity during rolling. This advantage is to a considerable extent negated by a complicated coupling method as described above.
The object of the invention is to provide a device which makes it possible to couple together thin-cast slabs, which have optionally been reduced preliminary, quickly and easily. This object is achieved by means of a device which is characterized in that a welding machine is arranged between the continuous-casting machine or continuous-casting machines and the rolling device, for joining slabs together.
With a welding machine, it is possible to quickly join together end faces, which are straight or have some other simple shape, of two slabs which are to be coupled to one another. A welding machine does not take up much space, so that the slabs which are to be joined together are only exposed to the atmosphere for a short time and therefore also only emit heat to the environment during a short time. Consequently, the use of a welding machine also contributes to reducing the amount of oxide which is formed on the surface of the slabs which are welded together.
In order to avoid interim stores, for example in the form of a coil box, a preferred embodiment of the device according to the invention is characterized in that the welding machine is displaceable along a welding length in the standard passage direction of the slabs through the device towards the rolling device. By allowing the welding machine to move along with the slabs which are to be welded together, the slab, whether or not it has been reduced in size, and the strip can run at the same speed throughout the device, taking into account the reduction in thickness.
A further embodiment of the device according to the invention is characterized in that the welding machine is displaceable in the standard passage direction of the slabs through the device towards the rolling device at a speed of between 4 and 20 m/min, preferably at a speed of between 10 and 17 m/min. In an endless rolling process, the speed at which the slab enters the rolling device is, depending on the final strip thickness which is to be achieved and on whether this final thickness is reached in the austenitic, ferritic or austenitic-ferritic mixed field, in the range between 4 and 20 m/min, more preferably in the range between 10 and 17 m/min. For the process to operate efficiently, the speed at which the welding machine is displaced is preferably equal to the speed, if appropriate taking into account a reduction in thickness, at which the slab is introduced into the rolling device.
A further embodiment is characterized in that the welding machine is an induction-welding machine. This prevents the need to introduce into the weld a welding material with a chemical composition which differs from the chemical composition of the slabs which are to be welded together. This is particularly important for low-alloyed steel grades, in particular IF steel grades. In addition, the output of an induction-welding machine is easy to control.
The transfer of heat from the slabs which have been welded together to the atmosphere is limited further by an embodiment of the device according to the invention which is characterized in that the welding machine is provided with means for limiting the transfer of heat from the slabs to the environment.
It has been found that, using slab thicknesses and rolling speeds which occur in practice, the process can be operated with success, even using multi-strand continuous-casting machines, with a furnace device which is characterized in that the total length of the furnace device is between 250 and 330 m.
The slabs which are to be welded together are moved into a desired position with respect to one another using positioning means, after which the slabs are welded together by means of the welding machine, because the positioning means and the movable welding machine cannot be fully accommodated in a furnace and it is inevitable that during welding the slabs which are to be welded together will cool down in the area of the weld. In order to produce the desired temperature homogeneity of the slabs, a further embodiment of the device according to the invention is characterized in that the furnace device comprises a first zone and a second zone which, as seen in the standard passage direction, are positioned one behind the other, and the welding machine is arranged between the first and the second zones. The furnace device preferably has means for conveying through slabs at an accelerated speed in order to be able to empty the furnace device quickly following an interruption to the process, whether planned or not, and before another interruption occurs.
It has been found that a good weld, with little cooling of the slabs, can be obtained in an embodiment of the device according to the invention which is characterized in that the first zone and the second zone are positioned at a distance apart which, measured in the standard passage direction, is 4-25 m, preferably 5-17 m. In order to return the slabs which have cooled down during the welding to the correct temperature, a second zone is positioned downstream of the welding machine, as seen in the standard passage direction, which second zone, according to the invention, is characterized in that it has a length of between 25 and 100 m. It has been found that, depending on the rate at which welding can be carried out and the welding length, sufficient temperature homogenization can be achieved with such a length.
In the second zone, the welded slab is to reach a temperature homogeneity which is desired for the subsequent rolling process. It has been found that a good level of homogeneity is achieved within the available time and length of the second zone in an embodiment of the device according to the invention which is characterized in that the second zone comprises a reheat-up section and a heat-through section. In order to minimize cooling during the welding process in which the slabs which are to be welded together are exposed to the environment, it is preferable that means for limiting the transfer of heat from the slabs to the environment to be arranged in the device, between the first zone and the second zone.
Current continuous-casting machines which are used in practice for casting thin slabs have a casting speed of approx. 6 m/min for a slab thickness of between 50 and 100 mm. For an endless rolling process, it is desirable for the speed at which the slab enters the rolling device to lie in the range between approx. 10 and approx. 20 m/min, preferably in the range between 12 and 16 m/min. In order to bridge the discrepancy between casting speed and desired speed of entry, it is proposed to use a multi-strand casting machine or a plurality of casting machines next to one another. In this case, it is preferable for the device to be provided with a second furnace device for accommodating a slab. In this case, there is a dedicated furnace device available for each casting machine or for each strand, and there is no need to include complicated transverse or longitudinal conveyor means for slabs in a furnace.
Currently, there are installations in use in which the abovementioned difference in speed between casting speed and speed of entry into the rolling device arises. This difference in speed may also arise in new installations or installations which are to be newly constructed, for example in cases in which, for whatever reason, a single casting machine or a single-strand casting machine is initially used. In the event of a new continuous-casting machine being subsequently installed or a second strand being added, it is preferable for at least one of the furnace device and the second furnace device to be provided with conveyor means for conveying a slab from the second furnace device to the furnace device.
In this case, the existing installation can be retained and a second furnace device is positioned in line with the new continuous-casting machine or the second strand. The conveyor means can be used to convey slabs from the second furnace device to the furnace device, after which they can be coupled together in the welding machine.
In connection with the limited space required, which is particularly important in a multi-strand casting machine, it is preferable for the conveyor means to comprise a so-called parallel ferry. An alternative is a swivel ferry, in which a slab section from the second furnace device is placed on the swivel ferry, the rear side of which is then rotated in the direction of the furnace device. The front side of the swivel ferry from the furnace device rotates towards the first swivel ferry mentioned, after which the slab section of one swivel ferry can be placed against the other swivel ferry. The swivel ferries then rotate back to their original positions. Advantages are simple connections to the media. A drawback is the increased amount of space which is required compared to a parallel ferry.
It has been found that rapid and successful temperature homogenization is achieved in an embodiment of the second furnace device which is characterized in that the second furnace device is provided with a second heat-up section and a second heat-through section, positioned downstream of the second heat-up section, as seen in the standard passage direction of the slabs.
In order to achieve rapid and successful temperature homogenization in the furnace device too, it is preferable for the furnace device to be provided with a first heat-up section and a first heat-through section, positioned downstream of the first heat-up section, on the entry side of the furnace device, as seen in the standard passage direction of the slabs.
In connection with achieving flexibility in the operation of the furnace device, inter alia in the event of or after a planned or chance interruption, it is preferable for the furnace device to be provided at the end, as seen in the standard passage direction, with a further heat-through section which is arranged downstream of the conveyor means, if present, and upstream of the welding machine.
The invention is also embodied by a process for producing a steel strip, in which liquid steel is cast in at least one continuous-casting machine to form a slab and, utilizing the casting heat, is conveyed through a furnace device and, in a rolling device, is rolled to form the strip with a desired final thickness. This process is also described in application PCT/NL97/00325. This application describes an endless process for producing a steel strip which has been rolled in the austenitic, ferritic or in the austenitic-ferritic mixed range. The process described provides a large number of advantages. One advantage for the ability to carry out the process is that individual slabs can be coupled together. The object of the invention is to provide a process for coupling slabs in such a manner that the process described can be carried out advantageously. This object is achieved by means of a process for coupling together slabs which is characterized in that slabs, which have optionally already been prereduced, are joined together by means of welding and slabs which have been welded together are rolled in an endless process in the rolling device. Coupling slabs by means of welding provides the advantage that the slabs can be quickly joined together without the formation of inhomogeneities in the chemical composition of the steel slab obtained.
In general, it will be necessary to carry out the welding on hot slabs which are temporarily outside the furnace device. Consequently, the slabs will inevitably cool down, during welding, at the location of the weld which is to be formed. In order to prevent temperature inhomogeneities occurring in the endless rolling process, a further embodiment of the process according to the invention is characterized in that the slabs, after they have been welded together, are temperature-homogenized at least at the location of the weld joint.
In the case of an endless rolling process, it is desirable for the steel to enter the rolling device at a relatively high speed. Present continuous-casting machines are unable to achieve a casting speed which corresponds to the desired speed of entry, if appropriate taking into account the reduction in thickness. Therefore, preference is given to a process according to the invention which is characterized in that slabs from two continuous-casting machines are welded together. With the aid of two or more continuous-casting machines it is possible to achieve a flow of slab material which is sufficiently great to be able to achieve the desired speed of entry into the rolling device.
An alternative which takes up less space and is easier to realize in particular in the case of new installations is characterized in that slabs from a multi-strand continuous-casting machine are welded together.
In the event that a plurality of continuous-casting machines or a multi-strand continuous-casting machine is used, it is advantageous for a plurality of furnace devices to be used simultaneously and for slabs from the furnace devices to be coupled together using the welding machine. In this case, a dedicated furnace device is available for each strand. The slabs from the furnace devices can be placed together, optionally in one of the furnaces, and then coupled to one another by means of welding.
When carrying out an endless rolling process, a large number of installation parts are coupled together by means of the steel slab or steel strip. An interruption at one of the installation parts means that the entire device, or a large part of the device, has to be shut down. This interruption may be unplanned or planned, for example in order to change rollers. In order to be able to cope with interruptions of whatever type, a further design of the process according to the invention is characterized in that the furnace device is used as a buffer space for the temporary storage of slabs in the event of interruption to one of the parts of the installation for processing slabs which have been welded together. The furnace device can act as a buffer both for interruptions to parts which are situated upstream and for interruptions to parts which are situated downstream. The longer the furnace device, the greater the buffer capacity will be.